/Engine/Source/Runtime/GeometryCore/Public/DynamicMeshEditor.h
Calculate and set the per-triangle normals of the two input quads. Average of the two face normals is used unless the quad is planar
the normal vector that was set
FVector3f ComputeAndSetQuadNormal ( const [FIndex2i](API\Runtime\GeometryCore\FIndex2i) & QuadTris, bool bIsPlanar )